I had the same problem with a seat belt on a sierra.
I had to disassemble the housing and refit a little spring.
The spring comes out of its place and donīt make its job.
this spring is the one in charge of returning back a little hook, as that hook is the one in charge of retaining the belt, so the spring makes the hook loosen the belt.
So if that spring itīs out of its place the hook is retaining the belt all the time.
When you open up the seat belt housing. you will have to brake a little bracket. But donīt worry, you will be able to close again the housing without that bracket.
And here it is, the seat belt runs again.
This happened to me with all the seat belts. I unbolt them to rebuild my car and one year later when i went to fit them again, all of them where looked.
Now they all work fine.
Hope this will help you
